1. Field of the Invention
The field of art to which this invention pertains is the solid bed adsorptive separation of phenylalanine. More specifically, the invention relates to a process for separating and recovering L-phenylalanine (hereinafter "phenylalanine") from an aqueous solution of phenylalanine and salts employing a zeolitic adsorbent to selectively adsorb phenylalanine.
2. Description of the Prior Art
Phenylalanine is an essential amino acid and is used in the synthetic production of pharmaceuticals and more recently extensively in the production of "Aspartame", a non-nutritive sweetener sold under the trade name "Nutrasweet". There are several routes to the production of phenylalanine: the fermentation of sugar; the enzymatic reaction of cinnamic acid; hydantoin or other sources, e.g., phenylacetaldehyde. All of these routes produce phenylalanine, which is a zwitterion at a pH of 6.5, together with other reaction products, such as lactic acid, acetic acid, phenyllactic acid, cinnamic acid and hydrocinnamic acid, salts, such as KCl, K.sub.2 SO.sub.4, (NH.sub.4).sub.2 HPO.sub.4, etc., sugars, other amino acids and organic acids.
In U.S. Pat. No. 4,584,400, a process for separating L-phenylalanine from a fermentation broth by a chromatographic process with non-polar adsorbents, e.g., XAD-2 and XAD-4 is disclosed, where the predominant contaminant is L-tyrosine. However, enormous volumes of water, the desorbent, are required to desorb phenylalanine.
Phenylalanine has also been separated from cinnamic acid, as disclosed in U.S. Pat. No. 4,604,483, utilizing XAD-2, XAD-4, XAD-7 and XAD-8 in the presence of at least 0.1N solution of a salt, e.g., ammonium chloride. In this process, the selectivity of the adsorbent for the two components is reversed due to the greater salting-out effect of ammonium chloride on the cinnamic acid than on the phenylalanine. Thus, phenylalanine is eluted first with substantially no cinnamic acid. Applicant's invention does not rely on the salting-out effect on the selectivity.
U.S. Pat. No. 3,787,317 discloses the use of at least two different chromatographic materials, e.g., dextran-based molecular sieves, usually crosslinked, to separate mixtures which are stated to include phenylalanine.
A technical bulletin (undated) promulgated by Rohm and Haas Company discusses the use of Amberlite XAD-7 in several separations, viz. fatty acids from water or toluene; phenol or m-chlorophenol from water or toluene; proteins from aqueous fluids of biological origin. One of these general suggestions for separations is more specifically disclosed in U.S. Pat. No. 4,616,078, wherein proinsulin-like substances may be separated from impure mixtures obtained by recombinant DNA methodology by adsorption on Amberlite XAD-7 or XAD-8 and elution with acetone or acrylonitrile under specified conditions.
The use of crystalline aluminosilicates to perform a number of separations is well known in the separation art. Examples of such separations are the use of zeolites to separate normal paraffins from branched chain paraffins, (U.S. Pat. No. 2,985,589), faujasites to separate olefinic hydrocarbons from paraffinic hydrocarbons (U.S. Pat. No. 3,265,750), zeolites to separate specific monosaccharides or classes of monosaccharides from carbohydrate feed mixtures (U.S. Pat. No. 4,024,331), etc. To my knowledge, none have been proposed for this separation and recovery of phenylalanine.
While crystalline aluminosilicates or zeolites have been used in adsorption separations of various mixtures in the form of agglomerates having high physical strength and attrition resistance, to our knowledge an effective chromatographic process for purifying phenylalanine has not been found. Methods for forming the crystalline powders into agglomerates are also known and include the addition of an inorganic binder, generally a clay of the kaolin type comprising silicon dioxide and aluminum oxide, to a high purity zeolite powder in wet mixture. The blended clay zeolite mixture is extruded into cylindrical type pellets or formed into beads which are subsequently calcined in order to convert the clay to an amorphous binder of considerable mechanical strength. Water permeable organic polymers or silica are also amorphous and may also be used as binders which do not require calcining. Suitable water soluble organic binders include cellulose esters, such as cellulose acetate or cellulose acetate butyrate, or cellulose nitrate as disclosed in Kulprathipanja Patents 4,248,737 and 4,295,994. The method of incorporating the binder in the zeolite is disclosed in said patents, which disclosures are incorporated herein by reference.
None of the references disclose an effective and economic chromatographic process for separating phenylalanine from a fermentation broth and/or from salts dissolved therein.
The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton U.S. Pat. No. 2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves shown in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipment utilizing these principals are familiar, in sizes ranging from pilot plant scale (DeRosset U.S. Pat. No. 3,706,812) to commercial scale and flow rates from a few cc's per hour to many thousands of gallons per hour.
The functions and properties of adsorbents and desorbents in a chromatographic separation of liquid components are well known, but for reference thereto, Zinnen et al. U.S. Pat. No. 4,642,397 is incorporated herein.